Twenty years ago, Queen Elizabeth II and President Francois Mitterrand braved the drizzle to cut the inaugural ribbon of the Channel Tunnel, realising a centuries-old dream of linking France and Britain under the sea.

A prodigious industrial adventure, the project mobilised 12,000 engineers, technicians and workers to create the world's longest underwater tunnel nearly 38km from northern France to southern England, earning it the "Global Engineering of the Century Award" by the International Federation of Consulting Engineers.

But this tour de force, officially inaugurated by the Queen and Mitterrand on May 6, 1994, was overshadowed for years by financial problems that almost tore apart Eurotunnel, the company contracted to manage and operate the tunnel until 2086.

At the end of 1987, before work on the tunnel kicked off, hundreds of thousands of eager, small shareholders bought Eurotunnel shares in the belief that these were solid, safe investments.

But colossal debt, disappointing traffic and quarrels between shareholders and management nearly sank the company over the years.

In 1987, shares were worth 35 French francs (the equivalent of 5.34 euros now). Fifteen years later, they were worth just a few centimes each.

The group even saw its stocks suspended for several months as it plunged deeper and deeper into the mire, until a restructuring deal was reached in 2007, paving the way for recovery.

Shareholders finally received their first-ever dividend in 2009, though it was worth just 4 centimes per share.

In 2013, Eurotunnel, which employs some 3,700 people, made a net profit of 101 million euros ($A151 million), prompting chief executive Jacques Gounon to say: "For the first time in the history of Eurotunnel, we think that the situation of the group is altogether satisfactory."

The idea to end Britain's isolation as an island and dig a tunnel to France emerged as early as the 18th century.

A first project was launched in the 1970s but was soon abandoned.

Then in January 1986, Mitterrand and British leader Margaret Thatcher officially signed an agreement to kick-start construction.

Thatcher faced considerable opposition to the project within her Conservative Party. But she pushed it through and famously insisted that "not a public penny" would be used.

As a consequence, the contract to conceive, make and put into service the tunnel was awarded to a consortium of ten British and French construction companies grouped under the name TransManche Link.

Construction lasted six years, cost some 15 billion euros and saw workers dig three tunnels – one for each direction and one in the middle for service work.

Vehicles can only cross the tunnel on board a rail shuttle, "as it is very difficult to ventilate a tunnel… Over a length of 50 kilometres, it's nearly impossible," said Michel Levy of the Setec engineering group, who worked on the project.

The huge, 1000-tonne tunnel boring machines that dug through the ground got off to a slow start on the French side due to difficult terrain and were slowed down by water infiltrations on the British side.

But finally, in a historic moment, a British and a French worker shook hands in December 1990 in the service tunnel, some 100 metres under the Channel.

The tunnel opened in 1994 and, six months later, the first Eurostar passenger train raced through.

After initial disappointing traffic, the number of people using the tunnel increasingly grew and some 330 million passengers have made the trip since 1994.

The tunnel, which carries passengers and freight traffic in separate services, has now become a formidable competitor to maritime ferry services and airlines on the Paris to London route.

A BRIEF HISTORY OF THE CHANNEL TUNNEL:

– February 12, 1986: The treaty of Canterbury launching the construction of the tunnel under the Channel is signed in the presence of then British prime minister Margaret Thatcher and French president Francois Mitterrand.

The result of a 200-year-old dream nurtured by both nations, the project involves a 50.5-km underwater link, the world's longest.

– November, 1987: Eurotunnel, the company entrusted with building the tunnel, enters the stock exchange. Hundreds of thousands of small investors in France and Britain rush to buy what are presented as safe investments.

Britain and France had agreed in 1986 that state aid would not be used for the tunnel.

– December 15, 1987: A tunneller starts to dig on the British side, in the direction of France. Digging on the French side starts in February 1988.

– December 1, 1990: A British and French workman knock down the last wall separating the two countries. The British workman, dressed in orange overalls with a yellow helmet and the French worker dressed in khaki and white helmet exchange a "bonjour" and a "welcome", to cheers.

– May 6, 1994: Inauguration of the tunnel by Britain's Queen Elizabeth II and Mitterrand at 100 metres below sea level. Dubbed by locals "the building site of the century" the tunnel was built at a cost of 100 billion francs (15.2 billion euros, $A22.75 billion), double what was forecast.

From 1994, Eurotunnel shares begin a long downward spiral, crippled by the group's debts and unattainable traffic forecasts.

– November 14, 1994: The first Eurostar high-speed train service for fare-paying passengers runs through the tunnel. The Paris-London journey takes three hours and six minutes – 20 minutes of which are spent in the tunnel – while the London-Brussels run takes three hours and fifteen minutes.

– November 26, 1997: Banks reach a financial restructuring plan, which transforms part of the colossal debt into shares. Eurotunnel thus avoids going into liquidation.

– April 7, 2004: Eurotunnel shareholders, who have lost most of their investments, vote to oust the board of the troubled operator, the first time individual investors have toppled the management of a major listed company in French history.

– September 11, 2008: a 1,000-degree inferno destroys part of the tunnel and limits its capacity for nearly five months.

– March 4, 2009: Eurotunnel announces that it will pay for the first time in its history a dividend, of 0.04 euros per share, to its 400,000 or so shareholders.

– January 22, 2014: Eurotunnel says it achieved one billion euros in turnover in 2013 for the first time. More than 20 million travellers have used the tunnel in 2013, and nearly 325 million since it opened.